1. Field of the Invention
The present invention is directed to a passive optical telecommunication system of the type having a central station which communicates via respective waveguides with a plurality of decentralized locations.
2. Description of the Prior Art
Recent developments in telecommunications technology have led to passive optical telecommunication systems wherein a plurality of decentralized stations (subscriber locations, or so-called distant units respectively combining a plurality of subscriber locations) are respectively connected via their own light waveguide subscriber line to an optical brancher. The optical brancher is connected directly or via at least one further optical brancher to a common light waveguide terminal of a central station--particularly an exchange--via a light waveguide bus. Such systems are described in European
Application 0 171 080; "Passive Fibre Local Loop for Telephone with Broadband Upgrade," Oakley et al., ISSLS'88, Conf. Papers pp. 9.4.1-9.4.5; and "The Provision of Telephony over Passive Optical networks," Hoppitt et al., BR Telecom Technol. J. Vol. 17 (1989) pp 100-113).
In such a passive optical telecommunication system, the signal transmission proceeding from the central station to the decentralized stations can proceed in a TDM (time division multiplexed) cell stream from which each decentralized station accepts only the cells intended for this decentralized station, and the signal transmission proceeding from the decentralized station to the central station can ensue in a TDMA (time division multiple access) method, in accord with which a decentralized station transmits each burst synchronized with the assistance of a delay means that is set in station-associated (dependent) fashion proceeding from the central station as described in German OS 4 016 359.
The signal transmission from the central station downstream to the decentralized stations and the signal transmission proceeding from a decentralized station upstream to the central station can thereby proceed in the same wavelength window (for example, common-frequency wavelength operation at 1.3 .mu.m).
Such passive optical telecommunication systems are known, for example, from IEICE Trans. Commun., Vol. E75-B (1992) No. 9. pp 841-848 or from the J. Dig. and Anal. Commun. Sys., Vol. 5 (1992), pp. 77-83), wherein the emitted light is intensity-modulated in the electro-optical transducer (laser diode) of the central station with a PSK-modulated pulse-like signal for signal transmission downstream from the central station to the decentralized stations and wherein the emitted light is intensity-modulated with a baseband signal in the electro-optical transducers (laser diodes) of each decentralized station for signal transmission upstream from a decentralized station to the central station.
A spectral separation of the signals of different transmission directions thus becomes possible in the electrical part of the receiver; disturbances due to increased shot noise, amplitude noise (RIN-Relative Intensity Noise) of the light source and a possible heterodyne effect are thereby not suppressed. Since a burst-like signal is present in the baseband in the upstream direction, special measures for a fast, time-dependent and amplitude-dependent response of the receiver of the central station are required in general, particularly in the case of different signal levels of the signals arriving in the central station proceeding from the individual decentralized stations. For example, the use of CMI channel coding is known for this purpose, however, this produces an elevated transmission speed (or modulation rate) and thus a need for increased bandwidth, which in turn results in a lower receiver sensitivity.